https://www.gamespot.com/articles/nobody-could-take-over-as-iron-man-avengers-infini/1100-6458283/

Avengers: Infinity War represents the culmination of the Marvel Cinematic Universe to date, going all the way back to 2008's Iron Man. That movie saw Robert Downey Jr. assume the role of Tony Stark--Iron Man--in one of those truly perfect actor-character pairings. Whether it's in Infinity War or further down the line, he'll eventually move on from the character, but those will be impossible shoes to fill, in the mind of Joe Russo.